Larval assemblages in intertidal habitats: the use of artificial and natural microhabitats

Abstract

Coastal habitats, and more specifically, intertidal habitats, host a unique range of biodiversity and are key areas for many fish and invertebrate species across one or more of their life stages. This is due to the provision of microhabitats which offer an escape from harsh environmental stressors and predation as well as increased food supply, hence increasing chances of survival. Due to the growing human population however, coastal habitats are being replaced by artificial structures (jetties, seawalls, piers, breakwaters) which partially or heavily fragment the natural environment through urbanisation-related expansion processes. These coastal infrastructures also have different physical properties from the natural environment and therefore tend to support different biological assemblages and can potentially alter the existing biodiversity and its functionality. The overall aim of this project was therefore to evaluate the use of artificial and natural intertidal microhabitats by fish and invertebrate larvae along the South African, Eastern Cape coastline. As independent case studies, fieldwork was conducted at an urban (Port Alfred Marina) and rocky shore (Kenton-on-Sea) site. Within each of these study sites, two replicated sheltered subsites were selected, which represented microhabitats. Samples were collected from these replicated microhabitats from September 2019 to February 2020 using light traps which targeted phototactic larval species, as well as a portable pump, for photo-neutral/negative taxa. All samples were preserved onsite in 99% ethanol and specimens were later counted and identified in the laboratory to the lowest possible taxonomic level using a stereomicroscope. Additionally, DNA barcoding was conducted on selected larval taxa for verification of morphological identification as well as contributing to the field of larval taxonomy through development of public database records. The barcoding technique was effective in positively identifying 96% and 58% of fish and invertebrate larvae sampled, respectively (overall identification success of 86%), to either family, genus or species level. Results of microhabitat use indicate higher larval abundances associated with artificial structures as compared to natural structures, with significant differences between the selected microhabitats within the rocky shores and the marina respectively, across months. High numbers of several early stage taxa were observed within the selected microhabitats in the marina, with Pinnotheres sp. (zoea) (Family: Pinnotheridae) being the most abundant invertebrate larval taxon collected at the artificial microhabitats of jetties and vertical walls. Fish larvae of Omobranchus woodi (preflexion) and Etrumeus whiteheadi (postflexion) were the most dominant at the selected artificial microhabitats within the marina. The DNA barcoding tool used in the current study to verify morphological identification proved to be instrumental in the accuracy of the reliable data collection of the early life stages present in these habitats. These results suggest that artificial structures may provide refugia for the vulnerable very early life stages of species and, in turn, play a potential facilitative role in reproductive and population connectivity which could result in replenishment of natural populations. It is therefore possible that these urban habitats could be considered as hubs for maintenance of coastal biodiversity.